Known in the art is a compression-vacuum action percussive machine (hammer drill), which comprises a casing accommodating a cylinder and a tool and also a striker with a transition cone on its end directed towards the tool, and a striker trapping device made in the form of a resilient ring and positioned in an annular groove cut in the casing wall (GB, B, 2085795).
This compression-vacuum action percussive machine has a disadvantage of low reliability of the resilient rings. To ensure trapping the striker when the machine is transferred to idling, the inner diameter of the trapping device should be smaller than the diameter of the front end of the striker. This difference in the diameters is responsible for increased friction between the striker and the trapping device and for braking the striker relative to the casing. The speed of the striker to be damped by the trapping device is as high as 6 to 10 m/s. The striker energy reduced by the trapping device in the radial direction (the direction of its deformation), wherein the impact loads are absorbed by the trapping device thus resulting in a premature failure thereof.
Also known in the art is a compression-vacuum action percussive machine (DE, C, 2756993), comprising a casing which houses a tool, a cylinder, wherein a drive piston and a striker are driven reciprocably, the striker being separated from the piston by an air cushion and having a chamfered end face engageable with the tool. Within the cylinder is also a striker trapping means formed by a carrier sleeve installed coaxially with the striker and having an inner wall with an annular groove cut therein, and a resilient ring mounted in the annular groove of the carrier sleeve.
In this percussive machine the striker is provided with an annular rib mounted after the chamfer, the diameter of the rib being larger than the inner diameter of the resilient ring, while the diameter of the striker body after the rib is smaller than the diameter of the resilient ring. Thus, the striker has a complicated configuration responsible for the concentration of stresses, resulting from heavy conditions of the striker operation. This configuration renders the percussive machine unreliable.
In addition, in the course of a percussive operation, the striker possesses considerable energy and, when the machine is transferred to idling, the striker spreads by its annular rib the resilient ring and passes therethrough in the direction of the tool. In case of a possible recoil, the striker has a considerable lower energy and the collar, formed due to the difference between the inner diameter of the resilient ring and the outer diameter of the annular rib of the striker, is able to retain the latter from further movement towards the piston. Thus, the striker is held by the resilient ring. To transfer the machine from the idling position into the percussion operation, a force should be applied to the machine body in direction of the face to be broken. When this is done, the tool thrusts with one end onto the face to be broken, while its other end presses against the striker, and overcoming the resistance of the resilient ring, forces the striker towards the piston. Due to rarefaction in the air cushion, the drive piston draws in the striker and the working cycle of the compression-vacuum action percussive machine begins. During the direct and reverse strokes of the striker, when the annular rib of the striker moves through the resilient ring, the latter undergoes substantial deformation and resulting damage.
It is the object of the present invention to provide a striker trapping means which will ensure a difference in friction forces with the striker moving in the trapping means in any direction.